Website:

• Modern version of website build with mkDocs available at exposnitc.github.io/expos-docs
• Added link to modern documentation build using mkDocs
• Docker based environment setup added

Website:
Add student feedback for 2020 batch.
Add bootstrap script that automatically pulls the latest version of code from the git repo.
Fixed a few typos.
Added link to Google Groups.
Created a repo to host user contributed content and added the link to the home page's sidebar.
Code:
Expl: Fixed linker error introduced by a breaking change in GCC10. Expl: Fixed strings containing colon getting parsed as labels.
xfs-interface: Allow specifying path to disk.xfs using --disk flag.

Setup instructions for Debian, Red Hat based distros and arch linux added

The definitions for User and Login have been added to the terminologies in the overview section of OS specifications.

The merge_sort.expl program to be used in assignment 5, stage 27 has been modified to work properly. Each process is made to wait for all the processes wth PID greater than itself inorer to prevent each process from closing leading to a reuse of PIDs.

The high level library interface contained a typo. The 3rd argument for read instruction was changed from Memory Address (Buffer) to variable name(to which data is to be read)

The roadmap contained typos in stage 7 and stage 14. In stage 7, logical pages for heap were changed from 3 and 4 to 2 and 3. In stage 14 a typo of OS startup code was changed to boot module.

The project was failing to make on some operating systems. This was due to a dependency issue on some systems and has been fixed in the new release.
The mem command in the XSM debug mode had an issue when called for a range of pages. It would just keep overwriting the contents of the mem file after writing a page and the mem file would contain the contents of only the last page in the range specified in the command call. Changes were made to the following directories:-

• xsm: The mem command was fixed and tested.

The roadmap contained a typo in the assignment question b of Stage 6. The memory locations 29714 in the question and 5160 in the answer has been corrected to 29706 and 51640 respectively.
The rm command of the xfs-interface encountered segmentation faults upon execution. Changes were made to the following directories:-

• xfs-interface: The rm command was fixed and tested.

NEXSM (a dual-core extension of XSM) is introduced. Stage 28 (Multi-Core Extension) describing how the OS can be ported to a two-core extension of the XSM machine has been added. The documentation for NEXSM architecture specification and eXpOS Design for NEXSM machine have been added. The changes in the sub-directories are:-

• expl: Library is modified to include INT 19.
• nespl: Three new instructions, start, reset and tsl, have been added.
• nexfs-interface: Commands for loading Bootstrap code for secondary core, software interrupt 19 and module 8-11 are added.
• nexsm: XSM machine is modified to simulate a two-core machine.

Stage 27 (Pager Module) has been modified. A swapper daemon is introduced as a new process similar to the idle program, with PID=15, and the Pager Module is called from the context of this process. To add fairness for the swapped out processes, if the TICK field of any swapped out process exceeds a threshold, that process is swapped in after swapping out another process in memory. The changes in the sub-directories are:-

• xsm: A few additional commands, including commands for displaying all the data structures, is added to the XSM debugger.

The changes in the sub-directories are:-

• expl: Modified the library (library.lib) to include INT 18 (Test0, Test1, Test2, Test3).

The Execption Handler has been modified to allocate 2 heap pages when a page fault for a heap page occurs. In Stage 27 (Pager Module), Get Free Page function in Memory Manager and Exit Process function in Process Manager has been modified to call Swap Out and Swap In function, respectively.

A new asignment problem has been added to Stage 27 (Pager Module).

A new step has been added to the algorithm for Login and Logout system calls regarding the current userID in the system status table. The algorithm for Fork system call has been modified to allocate heap pages for parent process (if not yet assigned already), so that parent and child processes will still have a shared heap after Fork.

The code for the Stage 22 assignment problem (Merge Sort) has been modified to avoid the stack overflow problem. A new assignment problem has been added to Stage 25 (File Write). The changes in the sub-directories are:-

• expl: Fixed a bug which caused an exception when function with more than one argument was called after an exposcall with more than one argument.
• xfs-interface: Fixed a bug which stored the file size of data files 1 more than the actual value.
• xsm: Fixed a bug in which the EIP value stored was 2 less than the actual value.

The Fork system call had a problem - if a process forks, the parent first gets a PCB entry and then calls Get Free Page function to allocate pages for Stack/User Area pages for the child. If the Get Free Page causes the parent process to change the state to WAIT_MEM, the PCB entry allocated for the child may also be given for another process. The problem has been fixed by modifying the Get Pcb Entry function in Process Manager. Two new assignments has been added to Stage 22 (Semaphores). The changes in the sub-directories are:-

• spl: The constant ALLOCATED has been added to the SPL pre-defined constants.

The algorithm for Release Page function in Memory Manager module has been modified to change the state of process from WAIT_MEM to READY when a free page is available. Makefile for myexpos directory is updated for make clean. The changes in the sub-directories are:-

• expl:
  a) Fixed a bug which caused return value of function to be overwritten when timer is called. The earlier version lowered the SP value before extracting return value from the call stack. If any interrupt occured in the meanwhile, the value in the stack was overwritten.
  b) Modified DIV instruction such that it doesn't call INT 10 when divisor is 0.
• spl: Earlier SPL version used the register R20 when large expressions were compiled. However, the machine had only registers up to R19. Hence, "Illegal register usage" was reported by the simulator. The SPL compiler is modifed to use only registers upto R19.
• xsm: Validity check is conducted for page/block number in load/store instructions.

The exception/interrupt handling documentation has been modified. The simulator will no longer push IP on top of stack when an exception occurs, thus the code for exception handler must be modified as per the new documentation. The changes in the sub-directories are:-

• expl: Added line numbers during compilation errors. Fixed read(p.x) bug. After compilation of <filename>.expl, the target code is stored as both assemblycode.xsm and <filename>.xsm.
• xfs-interface: Fixed long string bug which caused "Illegal instruction" when large strings were used.
• xsm: Machine won't push IP on top of the stack when an exception occurs, anymore. XSM debugger will now show privilege mode and IP along with next instruction. Added usertable and watchpoint in XSM debugger. Fixed the --timer 1 bug which caused the timer to be called infinitely when timer is initialised as 1.